A scoop-controlled fluid coupling comprises coaxially mounted impeller and runner elements, each in the form of a dished and vaned shell together defining a working circuit for working liquid, a reservoir casing rotatable with the impeller for holding working liquid radially outside the working circuit, a movable scoop having a scooping tip adjustable to different radial positions in the reservoir casing for collecting liquid from the rotating ring of liquid held therein and for returning it, optionally via a cooler, to the working circuit, from which the working liquid can return to the reservoir through one or more restricted outlets.
Such scoop-controlled couplings are often incorporated in drives for high inertia loads such as long conveyor belts for coal and other minerals. Such conveyor belts may be several kilometers in length but of relatively light belt construction. It is therefore essential that no excessive driving loads should be exerted on the belt, especially during start-up, which may take several minutes. Although a long conveyor belt can have two or more driving heads distributed along its length, and each drive may have two driving motors, each driving into a separate fluid coupling, there may be a requirement, under emergency conditions, for example failure of one or more motors or their supplies, for a reduced number of motors to be able to accelerate the load from rest to working speed without overstressing the belt or heating the fluid coupling beyond permissible limits.
In fact, it is now often a requirement for a coupling in such applications that the torque transmitted by the coupling throughout the start-up operation should not exceed 115% to 125% of the normal driving torque for that particular condition of conveyor loading.
Thus, when starting an empty or very lightly loaded conveyor, the load to be applied is small compared with the full motor load.
Where the drive or each drive comprises a squirrel cage motor and scoop-controlled coupling, the low starting torque is achieved by appropriately lengthening the starting operation. Thus the scoop-operating lever of the coupling is moved gradually over its full path of travel, in say three or more minutes during which time the coupling is operating with a decreasing but obviously large degree of slip with corresponding heat generation.
It has been found that the resulting expansion of the working liquid (particularly with synthetic liquids specified for coal mines) results probably in a greater mass of liquid being present in the working circuit than should be as determined by the position of the scoop, with the result in any event that the transmitted torque becomes greater than intended with risk of damage to the driven load or of overloading the motor or other prime mover.